The key performance criteria for nonwoven binders in medical-surgical fabrics include the ability to impart softness, or "hand", toughness, tear strength, compatibility with fluorocarbon additives which are used to enhance water repellency and cationic additives for antimicrobial activity, resistance to fabric blocking and low levels of free formaldehyde.
The current classes of polymer emulsion binders used in medical-surgical fabrics are based on flexible polymers like acrylates. Fabric softness correlates with the low glass transition temperature of the polymer. Toughness is enhanced by the incorporation of functional monomers that crosslink under garment fabrication.
Vinylacetate-ethylene copolymer emulslon binders of the prior art have not imparted a balance of softness and toughness for medical-surgical garments. Softness, or "hand", is the assessment of fabric feel via the sense of touch. Fabric softness and toughness are dependent on a combination of the chemical structure of the polymer binder and the type of stabilizer used in the emulsion polymerization. Vinyl acetate-ethylene copolymer emulsions are best stabilized by protective colloids and nonionic emulsifiers such as partially hydrolyzed polyvinyl alcohol, cellulosics, and alkylphenoxy polyalkoxy alcohols. These stabilizers also offer the required compatibility with the cationic additives.
In the case of polymer structure, attempts have been made to match the softness needs by maximizing the level of ethylene in the copolymer with vinyl acetate to lower the glass transition temperature (T.sub.g). This has improved the softness performance, but the stabilizer type has a contributory effect that either sacrifices the softness to gain toughness as in the case for polyvinyl alcohol or sacrifices toughness to maintain softness as in the case for cellulosics and alkylphenoxy polyalkoxy alcohols.
Thus, the industry has not accepted the use of a vinyl acetate-ethylene copolymer emulsion as a nonwoven binder for medical-surgical fabrics.
U.S. Pat. No. 3,094,500 discloses the emulsion homopolymerization of vinyl ester monomers in the presence of critical amounts of at least 10% of a low molecular weight 70-90% hydrolyzed polyvinyl ester having a viscosity of 1.2 to 3.0 centipoises for a 4% aqueous solution at 20.degree. C. The emulsions of the vinyl ester homopolymers produce films which are stated as having excellent remoistenability, clarity and gloss.
U.S. Pat. No. 3,692,723 discloses the aqueous dispersions of ethylene and vinyl acetate copolymers prepared by a copolymerization process utilizing a particular combination of nonionic emulsifier, anionic emulsifier and at least one high molecular weight protective colloid which may be a polyvinyl alcohol having a degree of polymerization of 400-2000 and containing 5 to 15% by weight of residual acetic acid groups.
U.S. Pat. Nos. 3,661,696; 3,734,819; and 3,769,151 disclose the copolymerization of ethylene and vinyl acetate in the presence of a preformed seed emulsion and a minor amount of a protective colloid comprising a mixture of fully and partially hydrolyzed polyvinyl acetate. Suitable fully and partially hydrolyzed polyvinyl acetates are described as having a viscosity of 5-125 centipoises and 2-80 centipoises, respectively, as 4% aqueous solutions at 20.degree. C.
U.S. Pat. No. 4,118,357 discloses an aqueous dispersion of a pre-crosslinked polyvinyl ester grafted onto polyvinyl alcohol.
U.S. Pat. No. 4,133,791 discloses preparation of an aqueous ethylene-containing copolymer dispersion prepared in the presence of from 1-15% by weight of polyvinyl alcohol with a degree of hydrolyzation of 70-100%.
U.S. Pat. No. 4,043,961 discloses an aqueous emulsion of vinyl acetate-ethylene copolymer prepared in the presence of protective colloid comprising fully hydrolyzed vinyl alcohol copolymer containing methyl methacrylate.